Pretreating metal substrates with phosphate conversion coatings and chrome-containing rinses has long been conventional for promoting corrosion resistance. To maximize corrosion resistance over steel substrates, cationic electrodeposition compositions are often formulated with lead as either a pigment or a soluble lead salt and are applied over pretreated (phosphated and chrome rinsed) substrates. Disadvantages associated with phosphating include the amount of plant space required for processing due to multiple (usually eleven to twenty-five) stages; high capital cost; and generation of waste streams containing heavy metals, requiring expensive treatment and disposal. Additionally, lead and chromium used in the electrodepositable composition can cause environmental concerns. The lead may be present in the effluent from electrodeposition processes and chromium may be present in the effluent from pretreatment processes, and these metals need to be removed and disposed of safely, which again requires expensive waste treatment processes.
To alleviate at least some of the foregoing disadvantages, alternative, non-phosphate based pretreatment compositions have been developed. For example, pretreatment compositions based on a group IIIB and/or IVB metal compound have recently become more prevalent. In addition, lead-free electrodepositable compositions containing yttrium have been developed that improve the corrosion resistance of the electrocoated metal substrate, particularly when the metal substrate is untreated. It is believed, however, that pretreatment compositions based on a group IIIB and/or IVB metal compound and lead-free electrodepositable compositions containing yttrium have not been used in combination.
As a result, it would be desirable to provide a method for providing a yttrium-containing coating on a substrate that utilizes a pretreatment composition based on a group IIIB or IVB metal compound. More particularly, it would be desirable to provide such a method wherein the resulting coating system, in at least some cases, exhibits excellent adhesion properties between the pretreatment composition and the yttrium-containing coating.